Noncontact seals are utilized to seal gas or the like. Noncontact seals include seal faces where high pressure gas is induced to keep the seal faces out of contact with each other. Such noncontact seals are provided at the sealing face with a very slightly tapered portion, a circle groove, a thin air flow hole or a spirally radial groove to induce high pressure gas to the sealing face thereof.
However it is hard (it increases the cost) to form the tapered portion, the groove or the hole with a high accuracy. In noncontact seals the seal faces are not completely noncontact; they contact each other slightly owing to vibration and/or poor accuracy of machining. Such contact causes wear of the sealing faces and deformation of the grooves follows. Thus the sealing ability of noncontact seals become worse as the sealing faces wear and therefore noncontact seals cannot keep providing stable and reliable sealing.
Furthermore the hole and the groove can sometimes be bigger than the required size because of difficulty of forming of small grooves, holes or the like and because of the need to make an allowance for the wear of sealing faces caused by vibration. Such a bigger groove and hole permits entry of more air, that separates the sealing faces too much and makes fine adjustment of the gap between the sealing faces difficult.